An optical waveguide device using an electrooptical crystal such as a LiNbO3 (LN) substrate, a LiTaO3 substrate or the like is being developed. For example, the optical waveguide device is used as a mach-zehnder optical modulator. A voltage of an operating point of the mach-zehnder modulator fluctuates according to environmental temperature or the like. Therefore, the voltage of the operating point may be kept constant if an output light is monitored. The output light may be monitored with use of an off-light. Japanese Patent Application Publication No. 05-53086 (hereinafter referred to as Document 1) discloses an art where a sub optical waveguide portion for guiding a radiated light (the off-light) is provided in addition to a waveguide in which a main signal (on-light) transmits.
However, in the art of Document 1, a sub optical waveguide portion having a sharp edge is used in order to guide the off-light to the sub optical waveguide portion efficiently. Manufacturing variation of the edge shape may be a problem in a manufacturing process. The variation of the edge shape of the guide may cause variation of an extinction ratio, monitor-light-receiving sensitivity or the like.